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Numerical analysis of geosynthetics and engineering fill in performance of reconditioned ballasted track
Over the past few decades, geosynthetics have been used extensively during track reconditioning to improve soil stability as they offer many advantages including cost effectiveness, ease of installation and minimal earthworks. Among the wide range of products in the market, geogrid remains the most commonly used geosynthetics for soil reinforcement. The aims of this paper are to investigate the effect of varying subgrade properties on track performance and to examine the effectiveness of geogrids and engineering fill for track reconditioning purposes. In the current study, numerical analyses were conducted using engineering software OptumG2, a finite element program for geotechnical stability and deformation analysis. The results of the parametric study indicated that geogrid inclusion within track substructure has considerable effect on settlement reduction and, in particular, increases the bearing capacity of railway track. The results also suggested that increase in axial stiffness of geogrids has minimal impact on track deformation. The most effective and practical location for geogrid reinforcement was achieved at interface between ballast and capping layers irrespective of the subgrade strength and stiffness. Sensitivity analyses showed that both total settlement and the bearing capacity of the railway track were most affected by the changes in the friction angle of subgrade, compared with cohesion and elastic modulus of subgrade, with or without geogrid reinforcement. The findings concluded that proper design of geogrid reinforcement can eliminate the need for or significantly reduce the thickness of engineering fill for ground improvement purposes.
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2018 Sydney Symposium
Advances in Site Investigations, Monitoring and Instrumentation
Peter K. Robertson and Michael Nicholson
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A new ring shear apparatus for the determination of the residual shear resistance of remoulded brown coal
Victorian brown coal is a typical intermediate geomaterial, whose behaviour falls between that of soft rock and engineering clay. The intact material exhibits lower permeability and higher tensile strength compared to overconsolidated clay. Large, shallow open cuts are used to mine the coal in the Latrobe Valley, Victoria, Australia, predominantly as fuel for Victoria’s power stations. For batter design the brown coal is treated as clay with high values of cohesion and friction angle. Composite rotational and block sliding is a recognised failure mode for this material and it is apparent from recent observations that failure risk increases with time. During long-term movements of the brown coal behind and below the batters it is anticipated that the material will be crushed and remoulded along sliding surfaces as a result of progressive pre-failure displacements. After periods of decades, the assumption is that for large sections along any incipient failure surface, shear strength will be at or close to the residual shear strength of the material. Thus. it is important to understand whether this assertion is correct and the processes leading to the weakened state. A new ring-shear test apparatus has been designed to determine the variation of shear strength of this material for a range of shear strains under essentially drained conditions. In this paper the design considerations and the resulting form of the test apparatus are presented. The equipment is also applicable to the testing of the interseam clays, silts and fine sands that are also found in the mines.
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Fibre Reinforced Soils For Geotechnical Infrastructure
This paper presents the results of recent laboratory studies on fibre reinforced soils. Drained and undrained triaxial test results highlight how soil stress-strain behaviour may be altered by mixing with discrete flexible fibres. In triaxial compression a considerable strength increase is induced by the presence of fibres, while in extension the strength increase is very limited. This is attributed to the fibre orientation distribution with respect to the tensile strains developed. Also presented in the paper is a framework for introducing the effects of fibres and their orientation into a constitutive model to describe the anisotropic stress-strain behaviour of fibre reinforced soils. Model simulations of selected test results are shown. Also described are examples of future investigations and trials required to make the soil reinforcement technology ready for use in industry.
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Numerical analysis of the bearing capacity of inclined loaded strip footings supported on sheet pile wall stabilized slopes
In practice, the bearing capacity of a strip footing adjacent to a slope is significantly reduced. This paper aims to control the generated failure mechanism and ameliorate the slope stability, using a sheet pile wall reinforcement technique. A two dimensional finite element limit analysis is used to examine the failure condition, through OptumG2 code. The effect of inclined loading on failure envelopes is investigated, through the average of upper and lower bound solutions. This paper focuses on the estimation of the undrained bearing capacity improvement factor for a given load inclination and slope angle. A new evaluation of the size and shape of failure envelopes is presented. Thus, a comparison between the undrained bearing capacity improvements before and after the sheet pile reinforcement is made, to study the most efficient case. The modification in failure loads are compared with those available in the literature.
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Uncertainty, economic risk analysis and risk acceptance criteria for mine subsidence
There are often differences in ‘expert opinions’ of what constitutes appropriate design parameters for determining the level of stability of the existing pillars of old coal mine workings. For example, there is seldom 100% certainty about the value of any design parameter. Instead there are varying degrees of confidence (or belief) for each possible design value. This uncertainty in design parameters can be represented by probability distributions. The inclusion of such probabilistic information into a probabilistic risk analysis will enable the probability of failure to be estimated. To illustrate the utility of risk analysis for decisions taken with uncertainty, a probabilistic risk analysis has been conducted to assess the uncertainty of design parameters on the level of stability of existing pillars within a disused coal seam beneath a proposed surface development in the Newcastle area. The case study considers dimensional and level of inundation uncertainties. This case study provides a preliminary framework for a risk-based approach to decision- making for a geotechnical system subject to high uncertainties. The outcomes of the risk analysis are probability of failure and annual economic risks (expected losses per year). The paper will describe the steps taken in the risk assessment, risk acceptance criteria and how results from a risk analysis may be interpreted by a decision-making development consent authority.
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On practical applications of the soil nail optimisation tool in cyclone recovery
Cyclones often inflict severe damage on soil structures, necessitating swift and effective recovery efforts. However, they also create multiple constraints for geotechnical engineering design solutions and construction, such as limited timeframes, and constrained budgets. Soil nailing reinforces unstable soil slopes, which have been crucial for restoring infrastructure post-disaster.
This paper investigates the practical applications of the Soil Nail Optimisation Tool (SNOT) developed by Michael Crisp (Crisp and Davies, 2024), particularly in the recovery work after Cyclone Gabrielle struck Auckland, New Zealand, in February 2023. The SNOT integrates Python coding and geotechnical data to facilitate the selection of soil nail lengths optimised for the site’s specific conditions. It reduces the time and cost of geotechnical design and, through optimisation, lowers construction costs. A case study based in the Waitākere Ranges in Auckland demonstrates how optimisation tools expedited recovery efforts while ensuring long-term stability. Furthermore, this paper discusses challenges and opportunities in implementing this tool, including cost considerations and technological advancements. This research underscores the significance of employing the SNOT in cyclone recovery projects, helping geotechnical engineers, disaster response agencies, and policymakers mitigate the impact of natural disasters on infrastructure and communities. Cyclones profoundly influence infrastructure and soil stability, posing significant challenges to disaster preparedness response and recovery efforts. Effective mitigation measures are essential for reducing vulnerability and enhancing resilience to cyclonic hazards.
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Soil-Structure Interaction Of Battered Minipile Groups In Sandy Soil
Battered minipile groups mimicking tree root networks have been gaining popularity as a footing solution for light structural applications in residential, commercial and infrastructure sectors, recently. Battered minipile group configurations are recently in the limelight due to advantages such as ease of installation and environmentally friendly nature. The lateral load resistance of battered minipile groups is investigated in this paper through a combination of physical and numerical modelling. Two-unconventional battered minipile groups with configurations representing the root network of trees with the capacity of engaging a larger volume of soil compared to conventional battered minipile group configurations are studied. A conventional battered minipile group is also included in the study to draw a direct comparison with the new minipile group configurations introduced in this paper. The conventional battered minipile group has two positively and two negatively 25° battered minipiles. The second type of group has one 25° perpendicularly battered minipile in the leading and trailing row each. Another unique orientation of the battered minipile group is also introduced in this study which has four diagonally outward 25° battered minipiles. The third type of minipile group with four diagonally outward battered minipiles offered the highest lateral resistance among the three groups. This better performance capability was attributed to the engagement of a larger volume of soil in resisting lateral load applied at the minipile head. Through this study, the industrial application of the unconventional minipile group configuration with better performance capability in terms of lateral load resistance can be advocated more confidently.
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A case study of deep excavation and retention design for Sydney Metro Northwest
This paper provides an overview of the Sydney Metro Northwest (formerly the North West Rail Link) project and the underground station excavation retention design and construction works, including the key requirements set out in the scope of works and technical criteria (SWTC). Based on the assessment of the geological conditions a soldier piled wall retention system was adopted for all five new underground stations and one of the two services facility shafts for ease and speed of construction. During the Castle Hill Station excavation a new planar wedge instability mechanism was considered to be credible based on the additional geological data, with the original three-dimensional block instability being no longer suitable. This led to redesign of the south wall stabilisation works based on the updated geological model and input parameters. The instrumentation and monitoring plan was also adjusted to ensure the required additional support provided would be adequate for the safety of the station box excavation. The monitored lateral movements at the capping beam and at the inclinometers were within the trigger values, indicating that the retention system constructed was robust.
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Innovative and sustainability considerations of contaminated materials for embankment construction of Coffs Harbour Bypass Project
This paper presents a case study of innovative and sustainability considerations of contaminated materials for embankment design and construction for the Coffs Harbour Bypass (CHB) project. The first consideration was development of a project specific Sustainability Management Plan (SMP) for CHB project to satisfy the Scope of Works and Technical Criteria (SWTC) requirements. The key themes outlined in the CHB SMP included: 1) climate change resilience; 2) whole of life impact and 3) design optimisation. To efficiently integrate sustainability considerations through all stages of design, relevant themes have been selected and allocated to each individual design package. The main considerations were presented through four case studies of the project regarding the encapsulation cell design for contaminated soils and insitu uncontrolled fill within the CHB project corridor. As the encapsulation cell material includes asbestos contaminated soil and building rubble, conventional construction compaction testing by nuclear densitometer was not considered practical. For this reason, a performance-based material placement and compaction methodology was specified. It is therefore expected that the settlement profile of an embankment containing encapsulation cells will be comparable to that of an embankment constructed wholly from general earth fill. A contaminated soil mound was developed onsite due to an excessive amount of contaminated soil for encapsulation and ease of construction. During construction the mound DCP profiling was introduced to validate the quality of compaction in lieu of plate load testing. The monitoring results of the constructed embankment are within the expectation without any surprises.